After installing Crystal Report 13.0 for Visual Studio 2010 from this link 

http://downloads.businessobjects.com/akdlm/cr4vs2010/CRforVS_13_0_2.exe 

and when you want to execute a page containing   Crystal Report Viewer's Control you may encounter the error saying CS0433: The type 'CrystalDecisions.Web.CrystalReportViewer' exists in both:' ......:




The cause is that that control references 2 differents versions of crystal report.So in your markup page you have :

<%@ Register Assembly="CrystalDecisions.Web, Version=10.5.3700.0, Culture=neutral, PublicKeyToken=692fbea5521e1304"
    Namespace="CrystalDecisions.Web" TagPrefix="CR" %>

The version of the assembly is 10.5 and It's not coherent with crystal report 13.0 installed.

The solution is to replace that registry version with this one:

<%@ Register Assembly="CrystalDecisions.Web, Version=13.0.2000.0, Culture=neutral, PublicKeyToken=692fbea5521e1304"
    Namespace="CrystalDecisions.Web" TagPrefix="CR" %>